The Layer 1 multiplexing and channel coding characteristics of the FDD (Frequency Division Duplex) mode of the UTRA are, at the priority date of this patent application, defined in the document “TS 25.212 V3.0.0 (1999-10), 3rd Generation Partnership Project (3GPP); Technical Specification Group (TSG) Radio Access Network (RAN); Working Group 1 (WG1); Multiplexing and channel coding (FDD)” available through the 3GPP. FIG. 1 illustrates the transport channel multiplexing structure for uplink as defined in said document. The functional blocks the serial connection of which is shown in the upper part of FIG. 1 are CRC attachment 101, transport block concatenation and code block segmentation 102, channel coding 103, radio frame equalisation 104, first interleaving 105, radio frame segmentation 106 and rate matching 107. Several entities of the above-described kind may be coupled to the inputs of a transport channel multiplexing stage 108, the output of which is further coupled to the serial connection of physical channel segmentation 109, second interleaving 110 and physical channel mapping 111.
In the downlink direction there may be certain modifications to the arrangement of functional blocks shown in FIG. 1, but at least the lower end of the arrangement which consists of the physical channel segmentation, second interleaving and physical channel mapping entities remains the same. For the purposes of the present invention it suffices to analyze the operation of the second interleaving and physical channel mapping blocks.
The aim of the second interleaving 110 is to permutate the bits in time domain so that bits that originally were close to each other in the bit stream to be transmitted are separated from each other in the time domain for the duration of their travel over the radio interface. This way a short interval of extremely bad interference conditions at the radio interface should not cause any bursts of several consecutive erroneous bits in the received and decoded bit stream. The second interleaving 110 takes place in inter-frame manner meaning that the data entity subjected to interleaving is one radio frame.
FIG. 2 illustrates the operation of the second interleaving stage. The bits that come as an input stream 201 to the interleaver are written into a bit array 202 which has a certain number of rows and a certain number of columns. The numbers shown in the input stream and the bit array are simply the serial numbers of the bits in the radio frame. Here the number of columns is shown to be 32, with column numbers ranging from 0 to 31. The columns are fed into an intercolumn permutator 203 which rearranges them into a different order. As examples, the 0th column remains 0th, the 17th column comes 1st, the 14th column comes 30th and the 31th column remains 31th after the intercolumn permutator 203. The bits are read from the permutated columns to the output of the second interleaving stage column by column. The bit stream 204 with the serial number of certain bits is shown as the output of the second interleaving stage.
The TDD or Time Division Duplex mode with its possibility of simultaneously using several spreading codes brings about some complications to the presented arrangement. If a single spreading code is used to transmit the bit stream, the bit stream 204 is transmitted by using that spreading code. However, in a multicode situation the transmitting device has at least two spreading codes at its disposal, and it transmits by using these parallel spreading codes simultaneously during a single time slot. The presently defined physical channel mapping arrangement is such that the parallel spreading codes are filled one at a time with bits taken from the bit stream 204. This may lead to the situation shown at the bottom of FIG. 2 where, during a certain time slot, e.g. bits 0 and 14, bits 32 and 46 and so on of a certain frame are transmitted simultaneously. Currently the number of parallel spreading codes may vary between 2 and 9.
The arrangement according to FIG. 2 has the drawback of in the multicode situation canceling much of the advantages usually obtained through the second interleaving, because certain bits that are near to each other in the frame are practically not separated at all in the time domain at the radio interface. The nature of the interference occurring in UTRA systems is such that it may occur e.g. that a part of a time slot either from the very beginning or from the very end of the time slot gets erased due to interference, especially inter-operator interference. The result of such an erasure, taken the arrangement of FIG. 2, is a burst of errors very close to each other in a received frame.